Local anaesthetics are commonly used to inhibit nociceptive pain, and are usually administered by local injection. Pharmaceutical compositions for local injection normally contain local anaesthetics at a concentration of 1 to 2%.
In the preparation of pharmaceutical compositions for topical administration it is preferred to have the local anaesthetic present at a higher concentration.
Local anaesthetics of the amide type, ATC code N01BB, are weak bases with a pKa of around 8. Consequently, in an aqueous solution at neutral pH these local anaesthetics are mostly present in their acid form. However, the acid form is charged and therefore less suitable to pass through biological membranes. In pharmaceutical compositions for topical administration it is therefore preferred to have the local anaesthetic present in its base form which can readily pass through biological membranes. This can be achieved by adjusting the pH of the pharmaceutical compositions to a pH around or even preferably above the pKa of the local anaesthetic, i.e. to a pH above 8 or higher.
However, this leads to problems of the base form of the local anaesthetics relating to poor solubility and stability in aqueous solutions.
This problem has been addressed for e.g. in EP 0833612 which discloses a pharmaceutical composition comprising an eutectic mixture of lidocaine base and prilocaine base. This mixture is in oil form at room temperature and can therefore be formulated as an emulsion. This eutectic mixture can only be obtained with a few local anaesthetic mixtures with suitable melting points, exemplified by lidocaine base and prilocaine base.
EP 1629852 describes a system where the local anaesthetic is kept in a solution at acidic pH and only mixed with a buffering solution with high pH shortly prior to use, providing a solution of the local anaesthetic at a pH between 5.5 and 7. In this pH interval only a small portion of the local anaesthetic is present in the base form, the form that readily penetrates membranes.
Despite many efforts of developing effective topical compositions of local anaesthetic agents, there still is need for a composition that safely and effectively can exert an anaesthetic effect at sites inside the body while meeting requirements of stability, sterility and a compliant administration procedure. For this purpose, the present inventors studied different thermogelling agents together with local anaesthetics of the amide type at a basic pH. The results indicated difficulties to find stability and to settle stable conditions even with suitable candidates of such thermogelling agents. The present invention aims at providing such stable sterilizable thermogelling pharmaceutical compositions comprising one or more local anaesthetics and at a concentration sufficiently high and at a sufficiently high pH to be able to provide effective pain relief following topical administration, while being easy to administer with conventional tools and sufficiently cohesive at the administration site to exert the anaesthetic effect in a safe, controlled and predetermined manner.